After a march through the capital, the men were provided with supper and housed at Soldier's Rest—part of a complex of buildings on the outskirts of the District of Columbia where soldiers camped, a Union hospital was established, and President Lincoln used a cottage as his retreat. On September 28 Forrest's company crossed the Chain Bridge over the Potomac into Virginia, then joined other Vermont units about four miles away at Camp Advance, which the Second and Third Vermont infantry had established while constructing a ring of earthen forts around Washington. In early October the four Vermont regiments (Second through Fifth) were transferred to the large Union encampment called Camp Griffin, where thousands of tents clustered around a hill west of the Chain Bridge less than two miles from the village of Lewinsville. There Forrest and the Vermont men settled in for a season of training and preparatory work among the newly-cleared, rolling hills of northern Virginia.

The Fifth Vermont's long stay at Camp Griffin gave Forrest plenty of time to write, and most of his surviving letters to his parents date from this time. Simple and direct, the letters have formulaic openings but proceed in an informal conversational tone and virtually without punctuation. They provide many details of the routines of camp life. The Army of the Potomac's chief, General George McClellan, excelled at training troops, and his soldiers' main business was to prepare for a Spring offensive. The men trained and drilled almost daily in the fall mud and winter snow, then suited up for weekly dress parades, usually on Sunday. Larger and more formal inspections were conducted once a month by General William Brooks, who took charge of the Vermont Brigade— the name given the Second through Sixth Regiments—in October. Brooks' brigade was one of eight in General W. F. "Baldy" Smith's division of the Army of the Potomac. On November 20 Forrest participated in a Grand Review of 75,000 troops attended by General McClellan, President Lincoln, and several cabinet members at Ball's Crossroads. Lincoln's secretary, John Nicolay, called it "the largest and most magnificent military review ever held on this continent."

Dress parades and grand reviews aside, camp life was dirty and dull. Like many infantrymen at Camp Griffin, Forrest rotated tasks, working in the cook tent, building fortifications, and serving on guard and picket details. Soldiers relieved their boredom by playing cards, gambling with dice, writing letters, and drinking whiskey— the last a practice that Forrest noted but promised his mother he would never indulge in. The men supplemented their rations by purchasing butter or tobacco at inflated prices from camp merchants, or "sutlers." More than once, Forrest called on a "pretty Virginian girl" who provided him with a home-cooked supper and a rare chance to socialize with the opposite sex. "I dident hardly know how to handle a knife and fork," an embarrassed Forrest confided to his parents.

Despite rumors of marching orders or an attack by Confederate troops camped nearby at Manassas and Centreville, the months went by with no battle and little real military action. Now and then Confederate and Union artillery harassed each other's camps by trading volleys from afar. Night alarms raised by Union pickets disrupted soldiers' sleep. Union officers periodically ordered early morning marches to assure their men's readiness. Kept on alert, the soldiers anticipated that Confederates would attack any day or—more to their liking—that Union troops would be summoned to begin the offensive toward Richmond. For the time being, they contented themselves with symbolic warfare and boasted naively of their prowess. Forrest reported firing his gun at a Confederate cavalryman who was scouting the Union position. Shortly after the Grand Review, Forrest's company went on a foraging expedition for hay, corn, and oats. "We would go right into the secesh hog pens and ran the hogs thrugh with our bayonets. The old Secesh would stand by and grate his teeth but it wouldent do him no good he had the bloody 5th to deal with."

Disease proved far deadlier than Confederate threats at Camp Griffin. Cold, wet, and huddled together in tents, soldiers who had never been exposed to measles or mumps were now infected. More serious diseases such as typhoid fever, malaria, and dysentery raged through the Vermonters' camp. As many as a fifth of the Vermont Brigade's five thousand men were stricken at one time, and nearly 200 died of disease between October and the middle of March. As yet unaffected, Forrest reported that at one point 34 men of his company had been on the sick list.

By the middle of February the situation at Camp Griffin had improved. Better sanitary measures were taken in camp, and infected soldiers were removed to central hospitals. With balmy weather returning, the mud began to dry and the men's health returned. News of Union victories at Roanoke Island, North Carolina, and Fort Donelson in Tennessee boosted morale. The Vermont recruits, eager to join the Union advances, were itching for a fight. Forrest wondered whether the Rebels would be defeated before the Vermont men ever left camp.

Pressure from President Lincoln and other Union officials mounted upon General McClellan to move his mighty army against the Confederates. Finally, on March 10th the Vermont Brigade left Camp Griffin and began marching southwest past Vienna toward the Confederate works at Manassas and Centreville. Now placed in McClellan's new Fourth Corps headed by General Erasmus Keyes, the Brigade was still commanded by General Brooks. Forrest and many Union soldiers expected that this "march for Manassas" would open an overland offensive, but Union advance pickets discovered that the rebels had fallen back and abandoned their fortifications near the Bull Run battlefield. The pause as Union officers pondered their next move gave Forrest an opportunity to survey the remains of the previous July's battlefield, where he gawked at still-unburied Union corpses and grabbed a souvenir insignia from a fallen Confederate.